In many laser projectors laser beams of the primary colors RGB are deflected by a two-dimensional deflection mirror (so-called scanner mirror) onto a projection surface. By appropriate movement of the scanner mirror and a correspondingly coordinated time-intensity variation of the laser sources, an image is generated on a projection plane (so-called “flying spot method”).
Diode lasers are used in miniaturized RGB laser projectors to generate the red and blue color fraction. Mostly infrared laser radiation is converted to green radiation by optically nonlinear methods to generate the green color fraction. Typical representatives are lasers of the DPSSL type (diode pumped solid state laser) or OPSL type (optically pumped semiconductor laser). Frequency conversion in the mentioned laser types is accomplished within the resonator or external to the resonator, depending on the version. Another possibility for generation of green laser radiation is the use of so-called upconversion fiber lasers. An energetically appropriate energy level that leads to generation of green laser radiation via stimulated emission is occupied in these lasers by means of 2-photon absorption of infrared laser radiation. Fiber lasers in the broadest sense can be classified among the DPSSL. In recent developments of fiber lasers, these can also be excited by means of blue laser diodes (400 nm-480 nm) (Sumita Company).
Based on the movement/time behavior, for example, a sine-like oscillation, of the scanner mirror in both axes, the equally large image spots are exposed for different lengths of time on the projection surface, depending on the position, viewed locally. For example, at a resolution of 640×480 pixels and scanning frequencies of fx=28 kHz in the x-direction and fy=1200 Hz in the y-direction, the exposure time of the image spots in the center of the image is about 17 ns. This is the duration of the shortest exposed image spot.
It follows from this that a time-intensity variation of the laser with defined bandwidth is necessary to achieve the required image resolution. For the example mentioned above, a necessary intensity variation bandwidth of at least 60 MHz is obtained. For higher resolutions or image repetition rates, a correspondingly higher intensity variation bandwidth is necessary.
Intensity variation bandwidths are possible with diode lasers (red and blue light) by variation of the operating current (subsequently called amplitude modulation) in the GHz range. For the green lasers, because of atomic lifetimes and optical travel times of the photons in the resonator, the amplitude modulation bandwidth is restricted. For miniaturized green OPSL, the amplitude modulation bandwidth lies in the range of the aforementioned 60 MHz. However, distinct signal distortions occur in the upper frequency range. The amplitude modulation bandwidth for miniaturized green DPSSL lies below 1 MHz. For these reasons, the green lasers are only operated comparatively rarely in amplitude modulation operation.
Green lasers are generally operated in continuous wave operation (continuously) and the intensity variation then occurs after the laser by means of an optically active switch (for example, an acousto-optic modulator (AOM) or an electro-optic modulator (EOM)). Such an arrangement has the drawback that the green laser is operated independently of image content and image spot position with maximum power absorption.